1. Technical Field
The present disclosure relates to an inkjet printing ink which has excellent jetting stability and is safe for living bodies.
2. Description of the Related Art
Inkjet printers have dramatically come into widespread use because of their advantageous properties including low noise and low running cost, and color printers which enable the printing on plain paper have also been provided increasingly on the market. However, in inkjet printers, it is very difficult to satisfy all of properties required for the inkjet printers, including color reproducibility in images, good image drying characteristics, prevention of blurring of characters or blurring of boundaries between colors and jetting stability. Therefore, the type of an ink to be used is selected on the basis of intended use or properties having higher priority.
For example, aqueous inks containing water as a main component, and also containing a coloring agent such as a dye and a pigment and a wetting agent such as glycerin are generally known. However, such aqueous inks may be insufficient with respect to image fixability depending on the types of substrates used.
In recent years, inks containing an organic solvent and therefore having high drying characteristics have also been provided. However, inks containing an organic solvent may be poor in safety for living bodies and the environment. Therefore, such inks are not suitable for practical applications for which safety for living bodies is required.
Meanwhile, for the purpose of diversifying the types of printable substrates, inks containing an ultraviolet (UV)-curable resin that can be cured by the irradiation with UV rays have been provided. However, in inks containing a reactive compound as mentioned above, there is a possibility that an unreacted compound remains, and the unreacted compound may affect living bodies and others. Therefore, the UV-curable inks are not suitable for practical application for which safety for living bodies is required, either.
In these situations, as inks having high safety for living bodies, edible inks have been proposed. For example, edible screen printing inks, each of which contains water, an alcohol, a surfactant, an edible insoluble inorganic micropowder having concealing properties, a thickening agent, a resin, an edible polymeric substance and the like, have been proposed (see, for example, PTL 1). Card-type cosmetics and the like, each of which is produced by pouring a suspension containing a cosmetic-use powdery material and an oily substance dispersed in a water-soluble binder, by screen printing have also been proposed (see, for example, PTL 2). However, these inks and suspensions have formulations suitable for a screen printing technique, and therefore it is difficult to use the inks as inkjet printing inks without any modification. Meanwhile, inks containing an edible dye have been reported as edible inks that can be used for inkjet printing (PTL 3).
However, in edible inks as disclosed in PTL 3, there is a problem that coloring materials are often deteriorated by the action of light or oxygen, and therefore the permanency of images formed from the inks is poor. Therefore, the fact is that an inkjet printing ink having all of requirements of safety for living bodies, permanency and jetting stability is not produced yet.